Question
What should be the minimum distance between source and reflector to hear the distinct echo of a simple sound?
Solution
The time period of persistence of ear for simple sound = 0.1s.
Therefore, to hear the distinct echo of simple sound, the minimum time taken by the sound to reach the listener after reflection should be 0.1s.
Let d be the distance between source and reflector.
The time taken by sound to reach the listener after reflection = t
Sound will travel the total distance of 2d.
∴ 2d = vt 
d = vt/2
Now, putting the values,
v = 340 m/s, t = 0.1s
We get,
d = 17 m
Therefore minimum distance of reflector from the source should be 17m.
